Of all 31 species of sharks and ocean rays, 24 are now threatened with extinction, several of which are classified as critically endangered – the most threatened category.
“It’s an incredible rate of decline, more pronounced than most elephant and rhino declines, and these animals are iconic in conducting land conservation efforts,” said Dulvey.
In the study, the researchers used two main indicators of biodiversity – the Red List Index, a list that measures the risk of extinction, and the Planeta Vivo Index, a data set that measures changes in population abundance – to map the rate of change of species populations over the decades.
Their results revealed “an alarming and continuing global decline in oceanic shark populations in the largest ecosystem in the world over the last half century, resulting in an unprecedented increase in the risk of extinction of these species,” the study said.
Factors such as human disturbances and climate change put pressure on these species – but overfishing is by far the biggest threat, and the relative pressure of fishing (which takes populations into account) has increased 18-fold since 1970, the study concluded.
Sharks, in particular, are caught for their meat, fins, gill plates and liver oil – they were hunted so intensely during the peak of overfishing in the early 2000s that between 63 million and 273 million sharks were killed each year, the study concluded.
Sharks are also particularly vulnerable to overexploitation due to their low population growth rates and long generation times – in some shark species it can take several years, even decades between the birth of an individual and the birth of its first offspring . The rays also faced rapid depletion and local extinction due to overfishing in their historic habitats.
There are some encouraging signs for specific species – the white shark, which saw a population drop in the 20th century, is now showing signs of recovery in several regions thanks to government bans and policies, the report said. Hammerhead sharks are also rebuilding their populations in the Northwest Atlantic, due to rigid quotas in the United States’ marine territories.
However, the threat of overfishing far outweighs any commercial regulation or sustainable fisheries management, the researchers warned.
Few countries have imposed specific catch limits for oceanic sharks, and even fewer have been able to rebuild populations that were destroyed by overfishing in the past century. Although governments have signed international treaties, poor implementation has failed to effectively restrict trade or retention of these species, the report said.
“It is something that policymakers can no longer ignore. Countries should work towards new international protections against sharks and lightning, but they can start immediately by fulfilling their internationally agreed obligations.”
The study called for immediate reform to “prevent the collapse of the shark population” and the potentially disastrous consequences for their ecological systems.
Specifically, the researchers urged governments to adopt catch limits for oceanic sharks that can support sustainable fishing and prohibitions on shark or stingray retention – actions that are crucial to saving these increasingly small populations “before depletion reaches a point without return”.